The present invention relates to a safety interlock and protection circuit used in a DC-AC inverter motor controller driving a permanent magnet motor, for example, driving the drum of a washing machine. The circuit according to the invention performs a door lock function and provides overvoltage protection and motor braking when the main motor controller fails and loses control while the permanent magnet motor is still spinning.
A permanent magnet motor has higher power density and higher torque per ampere than an induction motor. Induction motors have been commonly used in the past in washing machine applications. Permanent magnet motors have been used in more modem washing machines and have become an important power source for energy efficient appliances such as washing machines but also including air conditioners and refrigerators.
Permanent magnet motors are often driven by a DC-AC inverter circuit fed from an AC supply, that is the AC main supply is first converted to a DC voltage which is maintained across a DC bus and the DC bus voltage is then supplied to the inverter to be converted to an AC signal for each of the phases of the permanent magnet motor, typically three phases. A permanent magnet motor, unlike an induction motor, requires additional safety circuits and mechanisms, particularly in the washing machine application with its high speed drum operating in the spin mode.
A microcontroller and/or a DSP (digital signal processor) are common controller components to control the inverter power switches (IGBTs or FETs typically) to apply the desired voltage to the permanent magnet motor. When the controller fails and loses control, traditionally the solution to the problem of a runaway motor has been realized by a separate mechanical solution for the safety door lock and braking mechanism. This solution consists of a discrete relay, timer and crowbar switch with power transistors for overvoltage protection in conjunction with a mechanical brake. While this mechanical solution is achievable, it is prohibitively expensive and complex and also is subject to mechanical failure.
In the event of a controller failure, a permanent magnet motor exhibits two potentially dangerous conditions. One is the condition that a permanent magnet motor spins at high speed when coasting down. In this condition, if the door or the lid of the washer is opened, it creates a safety hazard of bodily injury if a person physically attempts to access the spinning drum. Therefore, it is essential that the door or lid must be locked while the motor spins and preferably the spin speed must be reduced to a stop condition (braked) as quickly as possible in the event of a controller failure.
The other condition is that a permanent magnet motor may generate overvoltage on the DC bus when the controller disengages from energizing the motor as a result of controller failure. This is due to the counter EMF (also known as back EMF) generated by the motor when it spins freely. This voltage can be particularly high when operated under a field weakening control to achieve a very high spin operation. If a failure occurs at this time, the voltage generated is quite high. The voltage is proportional to the product of motor speed (ω) and flux (Φ) generated by the permanent magnet as it moves with respect to the windings. In the field weakening mode, if the controller fails and is unable to control the motor, the weakened flux changes to the full amount of flux due to loss of flux weakening while the motor speed may be reaching more than three or four times its nominal operating speed depending on the spin mode speed. If no action is taken during such a controller failure during field weakening operation, the DC bus voltage, which is nominally rated at about 310 volts DC for a 230 volt AC RMS input main voltage, could reach approximately 1000 volts (three or four times the nominal DC bus voltage). This will result in damaging the power devices and high voltage ICs in the system since they are normally rated at 600 volts for a 230 volt AC RMS main input. The DC bus capacitor can also be damaged. Once damage occurs in the power system, it may no longer be possible to provide a safety door lock/braking mechanism since any circuits on the board likely will be damaged as well or be inoperative. Accordingly, it is necessary to 1) prevent such overvoltage in the event of controller failure, 2) quickly reduce the motor and drum speed and 3) provide a safety interlock to prevent physical injury.